In the present description the word tannin(s) identifies a class of products, the vegetable tannins, comprising natural products contained in several parts of trees and plants (leaves, fruits, barks, wood and roots). Tannins are a complex of organic compounds and it is difficult to characterise their various components, in any case the tannins to which the present invention refers can be defined as a mixture of polyphenols with molecular weights ranging between 500 and 3000 dalton and can be subdivided into two sub-groups: hydrolyzable tannins and condensed tannins (Kirk-Othmer “Encyclopedia of Chemical Technology” 2nd Ed. vol. 12, 319–324).
Even though the above mentioned tannins can be used as mordant agents for dyes, in the production of inks, in the oenologic and pharmaceutical fields, their primary use is in the tannage industry. In fact the most important feature of tannins is that they combine with collagen and other proteic substances contained in the animal skin, thus transforming it into leather. For such uses it is important that in the tannin extracts, the hydrolyzable tannins be present in very low amounts, preferably as impurities. Therefore the present invention is focused to obtain highly concentrated condensed tannin solutions.
The main sources of such tannins are: chestnut wood, quebracho wood, sumach leaves, wood and bark of some kinds of oaks, myrobalan.
The extraction process of tannins from vegetal products containing them is a traditional method and it is based on extraction with boiling water. The thus obtained solution is concentrated by evaporation. This method is known since the beginning of this century and substantially nothing has been modified, except for the traditional boiling pools, that have been replaced with counter-current extractors, which, in any case, do not allow higher extraction yields, in that the final tannin solution is still scarcely concentrated (about 5% by weight).
According to known techniques, as described in M. Giua “Trattato di Chimica Industriale” UTET, 45–49 (1973), chopped wood is loaded into a battery of autoclaves working in counter-current. In this manner fresh water meets the exhausted wood and the outgoing solution, enriched in tannin, solubilizes the tannin contained in high concentration in the fresh chopped wood incoming the battery of autoclaves. This treatment, called leaching, is carried out generally in water at about 110–120° C. and at a maximum pressure of (0.8 bar) 0.810−1 MPa for about 6 hours, with a water/wood ratio of about 2–2.4 by weight. Generally, through this boiling treatment a tannin solution at 4–5% by weight is obtained, with an extraction yield of about 60–65% (R. Jullien, Durand Ayme “Le tannnage vegetal”, Centre technique de cuir Ed., (1980). After clarification by settling, the tannin solution is concentrated in multiple evaporators, under vacuum to limit the tannin oxidation, up to the desired concentration (generally 40–50% by weight).
The thus obtained solution can be added up with known stabilising agents and stored, or it can be further treated, for example it can be powder dried, for example atomised.
The main drawbacks, that make unacceptable the above process, are the following: low extraction yields, too large amounts of water to be used, too high energy consumption to evaporate such water.
Several plants are known to perform the tannin extraction, however their main drawback is that the contact between the extraction fluid and the material to be extracted is unsatisfactory, thus making poor the plant performances.
The use of filtration membranes is known in this field, however the products obtained till now are unsatisfactory, both in terms of quality of the final product and/or in terms of the performances of the membranes.
The present invention solves the above problems.